(1) Field of the Invention
The present invention relates to boiling water reactors. The present invention includes a preferable reactor core lower structure for applying fuel assemblies of which four sides are surrounded with nuclear reactor control rods (hereinafter simply called control rods) blades into the reactor core.
(2) Description of the Prior Art
A schematic vertical cross section of a conventional boiling water reactor is indicated in FIG. 3. In the boiling water reactor, a region being surrounded with the shroud 2 in the nuclear reactor pressure vessel 1 and existing between the upper grid 4 and the lower grid 5, wherein a plurality of the fuel assemblies 3 are installed is called the reactor core 6. A portion lower than the lower grid 5 is called the lower plenum 7, wherein the control rod driving mechanism housing 8 and the control rod guide tubes 9 which attached to the control rod driving mechanism housing 8 are installed through a lower end plate of the nuclear reactor pressure vessel 1. A coolant existing in the circular flow path 10 between an internal wall of the nuclear reactor pressure vessel 1 and the shroud 2 is pressurized and sent into the lower plenum 7 by the internal pumps 11, and subsequently the coolant is sent into the fuel assemblies 3.
A structure which is installed in the lower plenum 7 is explained according to FIG. 4. The control rod guide tubes 9 are inserted by the control rod driving mechanism housing 8 which are welded to the lower end plate of the nuclear reactor pressure vessel 1, and stand perpendicularly. Furthermore, the fuel support piece 12 which can support four fuel assemblies 3 is inserted into the top portion of the control rod guide tubes 9. That means, a total weight of the four fuel assemblies 3 is supported by the lower end plate of the nuclear reactor pressure vessel 1 via the fuel support piece 12, the control rod guide tube 9, and the control rod driving mechanism housing 8. The lower grid 5 prevents the top portion of the control rod guide tube 9 from horizontal vibration.
The coolant supplied into the lower plenum 7 by the internal pumps 11 ascends along outer side wall of the control rod guide tubes 9, enters into the fuel support piece 12 through holes which are provided on the side wall near a top end of the control rod guide tube 9, and guided into the fuel assemblies 3. The coolant is not supplied so much into the control rod guide tubes 9, because heat generation of the control rods 13 (FIG. 4 indicates the control rods 13 under withdrawn condition) is small. That means, the control rod guide tube 9 has a role to limit the coolant flow paths. And, The entrance orifice 18 is provided at an entrance of the fuel support piece 12 in order to stabilize coolant distribution for each fuel assembly 3. Thus, the coolant flow ascends along outer side wall of the control rod guide tube 9, enters into the fuel support piece 12 by changing flow direction in 90 degrees, and by straining through the entrance orifice 18 alters to ascending flow again in the fuel support piece 12. The ascending flow is such a complex flow that it flows in a flow path expanding one-sided, and accordingly, pressure loss of the flow is large.
If a pressure loss in the fuel assembly 3 is remarkably larger than that of the flow before the entrance of the fuel assembly 3, a stability in two phase flow in the fuel assembly 3 is lost. Therefore, a large pressure loss by the fuel support piece 12 contributes to maintain the stability of the two phase flow in the fuel assembly 3, but a large delivery pressure is required for the internal pump 11.
The amount of the coolant supplied to the reactor core can be calculated by total summation of coolant flows of the internal pumps 11 which are obtained by calculation of the coolant flow for each pump based on pressure difference before and after the pump. Accordingly, an amount of coolant flow which enters into each fuel assembly 3 can not be confirmed.
Furthermore, when the internal pump 11 causes a trip, a margin in a critical power ratio (hereinafter indicates as a CPR, a larger CPR means larger safety), which is a ratio of an output power (critical power) at which nucleate boiling in the reactor core alters to film boiling to an operation power, becomes temporarily small because decrease in heat flux is slower in comparison with decrease in coolant flow in the reactor core.
In the control rod guide tube 9, the control rod 13 having cruciform blades is so provided as to move vertically. A partial cross sectional view of the control rod guide tube 13 downwards from top of the tube is illustrated in FIG. 5 which is a vertical cross section taken on line A--A in FIG. 4. In the FIG.5, it is indicated that the control rod 3 penetrates an approximately cruciform space which is formed by surrounding four flow paths 16 of the fuel support piece 12 guiding the coolant into the fuel assembly 3.
The velocity limiter 14 which has an umbrella shape as shown in FIG. 4 is provided at the lower end portion of the control rod 13. When the control rod 13 falls down (withdraw) fast from an insertion position, a large reactivity is supplied to the reactor core. The velocity limiter 14 is aimed at restricting falling speed of the control rod 13. That is, the falling of the control rod 13 is caused when the control rod 13 in the inserting condition is altered to the withdrawing condition, the coupling 15 for connecting the control rod 13 and the control rod driving rod 17 is detached by catching of the control rod 13 and only the control rod driving rod 17 alters to the withdrawing condition, and then the catching condition of the control rod 13 is canceled by any unknown reason. When the control rod 13 falls, water existing under the velocity limiter 14 moves from a small space between the control rod guide tube 9 and the velocity limiter 14 to a space on the velocity limiter 14, and flow resistance of the water moving restricts falling speed of the control rod 13.
Neutron absorbing capacity of the control rod 13 decreases depending on time elapsing, and the control rod 13 is exchanged after the absorbing capacity reaches to a predetermined value. An order of the control rod exchange procedure is explained hereinafter.
First, the four fuel assemblies 3 mounted on the guide tube 9 of the control rod 13 to be exchanged are hung up and removed, and subsequently the fuel support piece 12 is hung up and removed. Then, a space for hanging up the velocity limiter 14 is formed. The coupling 15 for connecting the control rod 13 and the control rod driving rod 17 is detached, and the control rod 13 is hung up and removed.
After exchange of the control rod 13 is finished, the fuel support piece 12 and the fuel assemblies 3 are piled up on the control rod guide tube 9 in the above describing order so as to return to the original condition.
A partial plan view of a conventional reactor core 6 is schematically indicated in FIG. 6, wherein one control rod 13 locates at the center and four fuel assemblies 3 are arranged around the control rod as previously described.
Accordingly, two sides of the fuel assembly 3 contact with the control rod 13. Besides, the dotted line in FIG. 6 indicates outer diameter of the control rod guide tube 9.
As previously described, it is desirable to make the pressure loss at the entrance portion of the fuel assembly small in view of pump operating power, but stability of the two phases flow in the fuel assembly must be maintained. Furthermore, it is desirable to prevent the temporary lowering of the CRP at the pump trip occurrence, and to achieve high burn up in order to improve fuel economy.
In order to improve fuel economy by achieving high burn up, a method wherein sizes of the fuel assembly and the control rod are made larger than before so as to optimize the ratio of water to uranium has been proposed as disclosed in JP-A-63-73192 (1988) and JP-A-63-261192 (1988). A partial plan view of the proposed reactor core is schematically illustrated in FIG. 7, wherein the control rods are so arranged as to contact with four sides of the large sized fuel assembly 3. However, if the above described arrangement of the fuel assembly 3 and the control rods 13 is applied to the lower structure of the conventional reactor core, adjacent control rod guide tubes 9 shown by dashed lines in FIG. 7 collide each other.
In order to solve the above described problem, structures comprising control rod guide tubes having a cruciform shape or a similar shape with the cruciform and fuel support pieces for two fuel assemblies are proposed in JP-A-63-83691 (1988) and JP-A-63-225189 (1988).
The cruciform control rod guide tube disclosed in JP-A-63-83691 (1988) makes it possible to avoid a mutual interference of the above described control rod guide tubes, but manufacturing of the tube is not simple. Especially, the control rod guide tube is required to have a necessary and an enough strength for supporting a plurality of fuel assemblies, and the cruciform control rod guide tube seems to be unreliable in view of mechanical strength. Further, the cruciform control rod guide tube has many bending portions which have small diameters for bending, and it is necessary to consider release of residual stress caused by manufacturing. Furthermore, a small gap must be retained along an inner side wall of the control rod guide tube for a long axial distance. If a portion covering the blades deforms inward at an intermediate portion of the total axial length, there will be a possibility that the portion causes friction with the blades.
The control rod guide tube having an almost similar shape with cruciform shape which is disclosed in JP-A-63-225189 (1988) has wider diameter and less thin portions than those of the cruciform control rod guide tube, and accordingly, a mechanical strength and avoidance of the mutual interference of the adjacent control rods guide tubes can be compatible. However, as the diameter of the control rod guide tube becomes wide, the coolant entrance portion of the fuel support piece which must be mounted on the control rod guide tube is arranged at inside of the control rod guide tube, and accordingly, the coolant is supplied to the fuel assembly through such a complex flow path that holes are provided in side walls at the upper end portion of the control rod guide tube, the coolant enters from side of the control rod guide tube, and the coolant flow is altered to an ascending flow. Consequently, the pressure loss of the coolant flow becomes large easily.
Main roles assigned to the lower structure of the reactor core are transferring whole weight of the fuel assemblies to the nuclear reactor pressure vessel and separating the coolant main flow which led to the fuel assemblies from the control rods. In view of pump operating power, the lower structure of the reactor core having a small pressure loss of the coolant to the fuel assemblies is desirable. The pressure loss relates to maintaining of two phase flow stability in the fuel assembly, and to temporary lowering of the critical power ratio (CPR) accompanied with flow decreasing at pump tripping. On the other hand, the reactor core having an arrangement wherein the control rods locate at four sides of the fuel assembly had disadvantages in a mechanical strength for supporting the fuel assemblies and in a pressure loss, although it had an advantage in improvement of fuel economy by high burn up of the fuel.